The field of photovoltaics generally relates to multi-layer materials that convert sunlight directly into DC electrical power. The basic mechanism for this conversion is the photovoltaic effect, first observed by Antoine-César Becquerel in 1839, and first correctly described by Einstein in a seminal 1905 scientific paper for which he was awarded a Nobel Prize for physics. In the United States, photovoltaic (PV) devices are popularly known as solar cells or PV cells. Solar cells are typically configured as a cooperating sandwich of p-type and n-type semiconductors, in which the n-type semiconductor material (on one “side” of the sandwich) exhibits an excess of electrons, and the p-type semiconductor material (on the other “side” of the sandwich) exhibits an excess of holes, each of which signifies the absence of an electron. Near the p-n junction between the two materials, valence electrons from the n-type layer move into neighboring holes in the p-type layer, creating a small electrical imbalance inside the solar cell. This results in an electric field in the vicinity of the metallurgical junction that forms the electronic p-n junction.
When an incident photon excites an electron in the cell into the conduction band, the excited electron becomes unbound from the atoms of the semiconductor, creating a free electron/hole pair. Because, as described above, the p-n junction creates an electric field in the vicinity of the junction, electron/hole pairs created in this manner near the junction tend to separate and move away from junction, with the electron moving toward the electrode on the n-type side, and the hole moving toward the electrode on the p-type side of the junction. This creates an overall charge imbalance in the cell, so that if an external conductive path is provided between the two sides of the cell, electrons will move from the n-type side back to the p-type side along the external path, creating an electric current. In practice, electrons may be collected from at or near the surface of the n-type side by a conducting grid that covers a portion of the surface, while still allowing sufficient access into the cell by incident photons.
Such a photovoltaic structure, when appropriately located electrical contacts are included and the cell (or a series of cells) is incorporated into a closed electrical circuit, forms a working PV device. As a standalone device, a single conventional solar cell is not sufficient to power most applications. As a result, solar cells are commonly arranged into PV modules, or “strings,” by connecting the front of one cell to the back of another, thereby adding the voltages of the individual cells together in electrical series. Typically, a significant number of cells are connected in series to achieve a usable voltage. The resulting DC current then may be fed through an inverter, where it is transformed into AC current at an appropriate frequency, which is chosen to match the frequency of AC current supplied by a conventional power grid. In the United States, this frequency is 60 Hertz (Hz), and most other countries provide AC power at either 50 Hz or 60 Hz.
One particular type of solar cell that has been developed for commercial use is a “thin-film” PV cell. In comparison to other types of PV cells, such as crystalline silicon PV cells, thin-film PV cells require less light-absorbing semiconductor material to create a working cell, and thus can reduce processing costs. Thin-film based PV cells also offer reduced cost by employing previously developed deposition techniques for the electrode layers, where similar materials are widely used in the thin-film industries for protective, decorative, and functional coatings. Common examples of low cost commercial thin-film products include water impermeable coatings on polymer-based food packaging, decorative coatings on architectural glass, low emissivity thermal control coatings on residential and commercial glass, and scratch and anti-reflective coatings on eyewear. Adopting or modifying techniques that have been developed in these other fields has allowed a reduction in development costs for PV cell thin-film deposition techniques.
Furthermore, thin-film cells have exhibited efficiencies approaching 20%, which rivals or exceeds the efficiencies of the most efficient crystalline cells. In particular, the semiconductor material copper indium gallium diselenide (CIGS) is stable, has low toxicity, and is truly a thin film, requiring a thickness of less than two microns in a working PV cell. As a result, to date CIGS appears to have demonstrated the greatest potential for high performance, low cost thin-film PV products, and thus for penetrating bulk power generation markets. Other semiconductor variants for thin-film PV technology include copper indium diselenide, copper indium disulfide, copper indium aluminum diselenide, and cadmium telluride.
Some thin-film PV materials may be deposited either on rigid glass substrates, or on flexible substrates. Glass substrates are relatively inexpensive, generally have a coefficient of thermal expansion that is a relatively close match with the CIGS or other absorber layers, and allow for the use of vacuum deposition systems. However, when comparing technology options applicable during the deposition process, rigid substrates suffer from various shortcomings during processing, such as a need for substantial floor space for processing equipment and material storage, expensive and specialized equipment for heating glass uniformly to elevated temperatures at or near the glass annealing temperature, a high potential for substrate fracture with resultant yield loss, and higher heat capacity with resultant higher electricity cost for heating the glass. Furthermore, rigid substrates require increased shipping costs due to the weight and fragile nature of the glass. As a result, the use of glass substrates for the deposition of thin films may not be the best choice for low-cost, large-volume, high-yield, commercial manufacturing of multi-layer functional thin-film materials such as photovoltaics.
In contrast, roll-to-roll processing of thin flexible substrates allows for the use of compact, less expensive vacuum systems, and of non-specialized equipment that already has been developed for other thin film industries. PV cells based on thin flexible substrate materials also exhibit a relatively high tolerance to rapid heating and cooling and to large thermal gradients (resulting in a low likelihood of fracture or failure during processing), require comparatively low shipping costs, and exhibit a greater ease of installation than cells based on rigid substrates. Additional details relating to the composition and manufacture of thin film PV cells of a type suitable for use with the presently disclosed methods and apparatus may be found, for example, in U.S. Pat. Nos. 6,310,281, 6,372,538, and 7,194,197, all to Wendt et al., and U.S. Pat. No. 8,062,922 to Britt et al.
As noted previously, a significant number of PV cells often are connected in series to achieve a usable voltage, and thus a desired power output. Such a configuration is often called a “string” of PV cells. Due to the different properties of crystalline substrates and flexible thin film substrates, the electrical series connection between cells may be constructed differently for a thin film cell than for a crystalline cell, and forming reliable series connections between thin film cells poses several challenges. For example, soldering (the traditional technique used to connect crystalline solar cells) directly on thin film cells exposes the PV coatings of the cells to damaging temperatures, and the organic-based silver inks typically used to form a collection grid on thin film cells may not allow strong adherence by ordinary solder materials in any case. Thus, PV cells often are joined with wires or conductive tabs attached to the cells with an electrically conductive adhesive (ECA), rather than by soldering.
However, even when wires or tabs are used to form inter-cell connections, the extremely thin coatings and potential flaking along cut PV cell edges introduces opportunities for shorting (power loss) wherever a wire or tab crosses over a cell edge. Furthermore, the conductive substrate on which the PV coatings are deposited, which typically is a metal foil, may be easily deformed by thermo-mechanical stress from attached wires and tabs. This stress can be transferred to weakly-adhering interfaces, which can result in delamination of the cells. In addition, adhesion between the ECA and the cell back side, or between the ECA and the conductive grid on the front side, can be weak, and mechanical stress may cause separation of the wires or tabs at these locations. Also, corrosion can occur between the molybdenum or other coating on the back side of a cell and the ECA that joins the tab to the solar cell there. This corrosion may result in a high-resistance contact or adhesion failure, leading to power losses.
Advanced methods of joining thin film PV cells with conductive tabs or ribbons may largely overcome the problems of electrical shorting and delamination, but may require undesirably high production costs to do so. Furthermore, all such methods—no matter how robust—require that at least some portion of the PV string be covered by a conductive tab, which blocks solar radiation from striking that portion of the string and thus reduces the efficiency of the system. As a result, there is a need for improved methods of interconnecting PV cells into strings, and for improved strings of interconnected cells. Specifically, there is a need for strings and methods of their formation that reduce interconnection costs and reduce the fraction of each PV cell that is covered by the interconnection mechanism, while maintaining or improving the ability of the cell to withstand stress.